surely something like this [url=http://cgi.ebay.com/ebaymotors/06-GTO]Ebay US motors LS6[/url] would do the job nicely. with a bit of fettling you would get up to 450bhp I reckon- with some money spent on the heads and cam you should be able to get to 500 naturally aspirated- no idea how tractable the engine would be though, with pretty low pressure forced induction you would easily see the other side of 500bhp.

oh yeah you can mate these engines to a porsche 5-speed transaxle and it would work very well- should be small enough to fit in just about any vehicle. It's the same setup as found in a Gardner douglas Lola T70 replica!

twin turbo Audi 4.2 V8 either second hand from an RS6 or take a old S6 and take it to MTM who will bolt a couple of turbos on so you get the same engine but for a bit less. Still really light but could be tuned for 600bhp quite happily and driveably.

I’m doing something similar to you, but I’ve stuck with the 4 pot Lancia engine, basically a Beta block with all steel internals, 16v head, GT35 BB turbo, dry sumped and Autronic management system – was good for over 400 at the wheels when it was in a Fiat 124!.

There are some guys putting a Lotus V8 twin turbo in a CAE Stratos replica – not sure how far they have got with it though. I’ve seen you over on stratossupersite.com – do a search for supercharge on there, I’m sure there was a discussion regarding S/C an Alfa V6.

a 5.5 rover V8 would be mucho pounds and not very efficient unless you go for an horrifically expensive wildcat setup- if you go V8 you might as well go LSx which is lighter, smaller and more powerful than an RV8

The Lsx isn't lighter then a rover but it's a similar size. A blown 4.0 rover or 4.6 could be viable but fitting it transversly may be a problem. A blown rover could be done on a reasonable budget. I agree that a 5.5 rv8 would still be quite expensive though and similar to lsx prices.

The mondeo may be on it's limits when blown to 400 odd hp unless you're prepared to do some work. There's a sleeve kit for that engine to increase capacity but I don't know how much that costs at the moment. I may explore that later.

How about using a 5 litre V12 BMW engine. For £5K you could buy the whole car, and quite a lot around. Dinan do an almost bolt on twin turbo that they want silly money for but the principal is there and that was pushing out 500bhp.